15,755 research outputs found
Incoherent Mollow triplet
A counterpart of the Mollow triplet (luminescence lineshape of a two-level
system under coherent excitation) is obtained for the case of incoherent
excitation in a cavity. Its analytical expression, in excellent agreement with
numerical results, pinpoints analogies and differences between the conventional
resonance fluorescence spectrum and its cavity QED analogue under incoherent
excitation.Comment: 4 pages, 3 figure
On the Evolution of the Cosmic Supernova Rates
Ongoing searches for supernovae (SNe) at cosmological distances have recently
started to provide a link between SN Ia statistics and galaxy evolution. We use
recent estimates of the global history of star formation to compute the
theoretical Type Ia and Type II SN rates as a function of cosmic time from the
present epoch to high redshifts. We show that accurate measurements of the
frequency of SN events in the range 0<z<1 will be valuable probes of the nature
of Type Ia progenitors and the evolution of the stellar birthrate in the
universe. The Next Generation Space Telescope should detect of order 20 Type II
SNe per 4'x 4' field per year in the interval 1<z<4.Comment: LaTeX, 19 pages, 3 figures, to be published in the MNRA
On Core Collapse Supernovae in Normal and in Seyfert Galaxies
This paper estimates the relative frequency of different types of
core-collapse supernovae, in terms of the ratio f between the number of type
Ib--Ic and of type II supernovae. We estimate f independently for all normal
and Seyfert galaxies whose radial velocity is <=14000 km/s, and which had at
least one supernova event recorded in the Asiago catalogue from January 1986 to
August 2000. We find that the ratio f is approx. 0.23+/-0.05 in normal
galaxies. This value is consistent with constant star formation rate and with a
Salpeter Initial Mass Function and average binary rate approx. 50 %. On the
contrary, Seyfert galaxies exceed the ratio f in normal galaxies by a factor
approx. 4 at a confidence level >= 2 sigma. A caveat is that the numbers for
Seyferts are still small (6 type Ib-Ic and 6 type II supernovae discovered as
yet). Assumed real, this excess of type Ib and Ic with respect to type II
supernovae, may indicate a burst of star formation of young age (<= 20 Myr), a
high incidence of binary systems in the inner regions (r <= 0.4 R25) of Seyfert
galaxies, or a top-loaded mass function.Comment: Accepted for Publication in MNRA
Cumulative physical uncertainty in modern stellar models. II. The dependence on the chemical composition
We extend our work on the effects of the uncertainties on the main input
physics for the evolution of low-mass stars. We analyse the dependence of the
cumulative physical uncertainty affecting stellar tracks on the chemical
composition. We calculated more than 6000 stellar tracks and isochrones, with
metallicity ranging from Z = 0.0001 to 0.02, by changing the following physical
inputs within their current range of uncertainty: 1H(p,nu e+)2H,
14N(p,gamma)15O and triple-alpha reaction rates, radiative and conductive
opacities, neutrino energy losses, and microscopic diffusion velocities. The
analysis was performed using a latin hypercube sampling design. We examine in a
statistical way the dependence on the variation of the physical inputs of the
turn-off (TO) luminosity, the central hydrogen exhaustion time (t_H), the
luminosity and the helium core mass at the red-giant branch (RGB) tip, and the
zero age horizontal branch (ZAHB) luminosity in the RR Lyrae region. For the
stellar tracks, an increase from Z = 0.0001 to Z = 0.02 produces a cumulative
physical uncertainty in TO luminosity from 0.028 dex to 0.017 dex, while the
global uncertainty on t_H increases from 0.42 Gyr to 1.08 Gyr. For the RGB tip,
the cumulative uncertainty on the luminosity is almost constant at 0.03 dex,
whereas the one the helium core mass decreases from 0.0055 M_sun to 0.0035
M_sun. The dependence of the ZAHB luminosity error is not monotonic with Z, and
it varies from a minimum of 0.036 dex at Z = 0.0005 to a maximum of 0.047 dex
at Z = 0.0001. Regarding stellar isochrones of 12 Gyr, the cumulative physical
uncertainty on the predicted TO luminosity and mass increases respectively from
0.012 dex to 0.014 dex and from 0.0136 M_sun to 0.0186 M_sun. Consequently, for
ages typical of galactic globular clusters, the uncertainty on the age inferred
from the TO luminosity increases from 325 Myr to 415 Myr.Comment: Accepted for publication in A&
On the age of Galactic bulge microlensed dwarf and subgiant stars
Recent results by Bensby and collaborators on the ages of microlensed stars
in the Galactic bulge have challenged the picture of an exclusively old stellar
population. However, these age estimates have not been independently confirmed.
In this paper we verify these results by means of a grid-based method and
quantify the systematic biases that might be induced by some assumptions
adopted to compute stellar models. We explore the impact of increasing the
initial helium abundance, neglecting the element microscopic diffusion, and
changing the mixing-length calibration in theoretical stellar track
computations. We adopt the SCEPtER pipeline with a novel stellar model grid for
metallicities [Fe/H] from -2.00 to 0.55 dex, and masses in the range [0.60;
1.60] Msun from the ZAMS to the helium flash at the red giant branch tip. We
show for the considered evolutionary phases that our technique provides
unbiased age estimates. Our age results are in good agreement with Bensby and
collaborators findings and show 16 stars younger than 5 Gyr and 28 younger than
9 Gyr over a sample of 58. The effect of a helium enhancement as large as Delta
Y/Delta Z = 5 is quite modest, resulting in a mean age increase of metal rich
stars of 0.6 Gyr. Even simultaneously adopting a high helium content and the
upper values of age estimates, there is evidence of 4 stars younger than 5 Gyr
and 15 younger than 9 Gyr. For stars younger than 5 Gyr, the use of stellar
models computed by neglecting microscopic diffusion or by assuming a
super-solar mixing-length value leads to a mean increase in the age estimates
of about 0.4 Gyr and 0.5 Gyr respectively. Even considering the upper values
for the age estimates, there are four stars estimated younger than 5 Gyr is in
both cases. Thus, the assessment of a sizeable fraction of young stars among
the microlensed sample in the Galactic bulge appears robust.Comment: Accepted for publication in A&A. Abstract shortene
Mixing-length estimates from binary systems. A theoretical investigation on the estimation errors
We performed a theoretical investigation on the mixing-length parameter
recovery from an eclipsing double-lined binary system. We focused on a syntetic
system composed by a primary of mass M = 0.95 Msun and a secondary of M = 0.85
Msun. Monte Carlo simulations were conducted at three metallicities, and three
evolutionary stages of the primary. For each configuration artificial data were
sampled assuming an increasing difference between the mixing-length of the two
stars. The mixing length values were reconstructed using three alternative
set-ups. A first method, which assumes full independence between the two stars,
showed a great difficulty to constrain the mixing-length values: the recovered
values were nearly unconstrained with a standard deviation of 0.40. The second
technique imposes the constraint of common age and initial chemical composition
for the two stars in the fit. We found that values match the
ones recovered under the previous configuration, but values are
peaked around unbiased estimates. This occurs because the primary star provides
a much more tight age constraint in the joint fit than the secondary. Within
this second scenario we also explored, for systems sharing a common
, the difference in the mixing-length values of the two stars only
due to random fluctuations owing to the observational errors. The posterior
distribution of these differences was peaked around zero, with a large standard
deviation of 0.3 (15\% of the solar-scaled value). The third technique also
imposes the constraint of a common mixing-length value for the two stars, and
served as a test for identification of wrong fitting assumptions. In this case
the common mixing-length is mainly dictated by the value of .
[...] For less than half of the systems can be
recovered and only 20% at .Comment: Abstract abridge
Evolution of the habitable zone of low-mass stars. Detailed stellar models and analytical relationships for different masses and chemical compositions
We study the temporal evolution of the habitable zone (HZ) of low-mass stars
- only due to stellar evolution - and evaluate the related uncertainties. These
uncertainties are then compared with those due to the adoption of different
climate models. We computed stellar evolutionary tracks from the pre-main
sequence phase to the helium flash at the red-giant branch tip for stars with
masses in the range [0.70 - 1.10] Msun, metallicity Z in the range [0.005 -
0.04], and various initial helium contents. We evaluated several
characteristics of the HZ, such as the distance from the host star at which the
habitability is longest, the duration of this habitability, the width of the
zone for which the habitability lasts one half of the maximum, and the
boundaries of the continuously habitable zone (CHZ) for which the habitability
lasts at least 4 Gyr. We developed analytical models, accurate to the percent
level or lower, which allowed to obtain these characteristics in dependence on
the mass and the chemical composition of the host star. The metallicity of the
host star plays a relevant role in determining the HZ. The initial helium
content accounts for a variation of the CHZ boundaries as large as 30% and 10%
in the inner and outer border. The computed analytical models allow the first
systematic study of the variability of the CHZ boundaries that is caused by the
uncertainty in the estimated values of mass and metallicity of the host star.
An uncertainty range of about 30% in the inner boundary and 15% in the outer
one were found. We also verified that these uncertainties are larger than that
due to relying on recently revised climatic models, which leads to a CHZ
boundaries shift within 5% with respect to those of our reference scenario. We
made an on-line tool available that provides both HZ characteristics and
interpolated stellar tracks.Comment: Accepted for publication in A&A, abstract abridge
Uncertainties on the theoretical predictions for classical Cepheid pulsational quantities
The expected distribution of Cepheids within the instability strip is
affected by several model inputs, reflecting upon the predicted
Period-Luminosity relation. On the basis of new and updated sets of
evolutionary and pulsational models, we quantitatively evaluated the effects on
the theoretical PL relation of current uncertainties on the chemical abundances
of Cepheids in the Large Magellanic Cloud and on several physical assumptions
adopted in the evolutionary models. We analysed how the different factors
influence the evolutionary and pulsational observables and the resulting PL
relation. As a result, we found that present uncertainties on the most relevant
H and He burning reaction rates do not influence in a relevant way the loop
extension in temperature. On the contrary, current uncertainties on the LMC
chemical composition significantly affect the loop extension and also reflect
in the morphology of the instability strip; however their influence on the
predicted pulsational parameters is negligible. We also discussed how
overshooting and mass loss influence the ML relation and the pulsational
parameters. In summary, the present uncertainties on the physical inputs
adopted in the evolutionary codes and in the LMC chemical composition are
negligible for the prediction of the main pulsational properties; the inclusion
of overshooting in the previous H burning phase and/or of mass loss is expected
to significantly change the resulting theoretical pulsational scenario for
Cepheids, as well as the calibration of their distance scale. These systematic
effects are expected to influence the theoretical Cepheid calibration of the
secondary distance indicators and in turn the resulting evaluation of the
Hubble constant.Comment: accepted for publication on A&
Theoretical investigation on the mass loss impact on asteroseismic grid-based estimates of mass, radius, and age for RGB stars
We aim to perform a theoretical evaluation of the impact of the mass loss
indetermination on asteroseismic grid based estimates of masses, radii, and
ages of stars in the red giant branch phase (RGB). We adopted the SCEPtER
pipeline on a grid spanning the mass range [0.8; 1.8] Msun. As observational
constraints, we adopted the star effective temperatures, the metallicity
[Fe/H], the average large frequency spacing and the frequency of
maximum oscillation power . The mass loss was modelled following
a Reimers parametrization with the two different efficiencies and
. In the RGB phase, the average error owing only to observational
uncertainty on mass and age estimates is about 8% and 30% respectively. The
bias in mass and age estimates caused by the adoption of a wrong mass loss
parameter in the recovery is minor for the vast majority of the RGB evolution.
The biases get larger only after the RGB bump. In the last 2.5% of the RGB
lifetime the error on the mass determination reaches 6.5% becoming larger than
the random error component in this evolutionary phase. The error on the age
estimate amounts to 9%, that is, equal to the random error uncertainty. These
results are independent of the stellar metallicity [Fe/H] in the explored
range. Asteroseismic-based estimates of stellar mass, radius, and age in the
RGB phase can be considered mass loss independent within the range () as long as the target is in an evolutionary phase preceding the
RGB bump.Comment: Accepted for publication in A&
A Radial Velocity Survey for LMC Microlensed Sources
We propose a radial velocity survey with the aim to resolve the current
dispute on the LMC lensing: in the pro-macho hypothesis the lenses are halo
white dwarfs or machos in general; in the pro-star hypothesis both the lenses
and the sources are stars in various observed or hypothesized structures of the
Magellanic Clouds and the Galaxy. Star-star lensing should prefer sources at
the backside or behind the LMC disc because lensing is most efficient if the
source is located a few kpc behind a dense screen of stars, here the thin disc
of the LMC. This signature of self-lensing can be looked for by a radial
velocity survey since kinematics of the stars at the back can be markedly
different from that of the majority of stars in the cold, rapidly rotating disc
of the LMC. Detailed simulations of effect together with optimal strategies of
carrying out the proposed survey are reported here. Assuming that the existing
30 or so alerted stars in the LMC are truely microlensed stars, their
kinematics can test the two lensing scenarios; the confidence level varies with
the still very uncertain structure of the LMC. Spectroscopy of the existing
sample and future events requires about two or three good-seeing nights per
year at a 4m-8m class southern telescope, either during the amplification phase
or long after.Comment: minor changes of text, ApJ accepte
- …